Images of diamond anvil cells courtesy of High Pressure Diamond Optics, Inc.
This is a photo taken through a microscope down the axis of a diamond anvil cell with transmitted light. The black edges of the photo are caused by the steel gasket. A hole in the gasket about 350 microns in diameter lets the light through. In the center of the hole is an acmite crystal at 10 GPa pressure. The crystal at the bottom of the hole is a ruby that is used to determine pressure. The frequency of fluorescence lines emitted by the ruby vary as a function of pressure.